Optical filters have been widely used for a long period of time to control both the intensity and spectral distribution of light. It is known that ancient civilizations had mastered the techniques of fabricating a variety of colored glasses and that they employed these colored glasses for purposes of blocking undesirable rays of the sun as well as for fabricating aesthetically pleasing objects. Optical filters continued to gain in populartity through the years as the uses therefor increased. Said filters are presently widely employed in a variety of applications; for example, the photographic and printing arts rely heavily upon the use of optical filters to balance the hue and intensity of light employed in those processes. Optical filters have also gained widespread acceptance in the manufacture of eye glasses for purposes of preventing glare.
Tinted glass may be considered to be one particular type of an optical filter. Tinted glass is currently finding increasingly greater usage as a material from which to fabricate automobile windows and architectural glazing panels insofar as the light transmission qualities thereof may be selected and specifically tailored so as to minimize the passage of heat and glare therethrough, thereby preventing harmful infrared and ultraviolet rays from being transmitted through the glass and into buildings and automobiles.
FIG. 1 is a graphic representation of the solar spectrum showing the relative intensity of solar radiation incident upon the surface of the earth as a function of the wavelength of that radiation. The abscissa of the graph is calibrated in nanometers and depicts a range of wavelengths between approximately 350 to 950 nanometers. Also indicated on the ordinate are those regions of the electromagnetic spectrum commonly denominated as ultraviolet, visible and infrared wavelengths. The exact boundaries between these various regions of the spectrum are somewhat subjective; however, for purposes of discussion herein the ultraviolet region shall be considered to be wavelengths shorter than about 400 nanometers and the infrared region shall be considered to be those wavelengths longer than about 800 nanometers with the visible portion of the spectrum spanning the range of 400-800 nanometers.
The graph of FIG. 1 also indicates the color perceived by the eye for various ranges of wavelengths of the spectrum; for example, wavelengths of approximately 450 nanometers are generally perceived as blue, wavelengths of approximately 525 nanometers appear green and wavelengths in the vicinity of 700 nanometers are generally seen as red. It will be noted that the relative intensity of the solar spectrum varies as a function of wavelength, this variation being due to the characteristic output spectrum of the sun as well as atmospheric filtering effects.
Although well known, it will be helpful for the purpose of understanding the discussion which follows that the energy associated with a particular photon will vary in inverse proportion to that photon's wavelength and may be determined by multiplying its frequency by Planck's constant. For example, photons of a wavelength of 400 nanometers have an energy of approximately 3.10 electron volts whereas photons of a wavelength of 800 nanometers have an energy of approximately 1.55 electron volts. Accordingly, it may be seen that ultraviolet photons, even those in the near ultraviolet region of 400 nanometers are quite energetic. Furthermore, and as can also be gleaned from FIG. 1, the relative intensity of these 400 nanometer photons is fairly high. For this reason it is not surprising that solar radiation can cause damage to a variety of materials, such as the interiors of automotive vehicles and buildings, as well as to human tissues such as retinas and skin. Accordingly, one object of using light filters is to eliminate the harmful ultraviolet wavelengths of the solar spectrum.
Since ultraviolet photons have sufficient energy (as shown hereinabove) to break many chemical bonds, particularly covalent bonds; it is not at all surprising that many materials are damaged by ultraviolet radiation. For example, paints, plastics or other organic materials are readily degraded by ultraviolet radiation, this degradation being manifested by yellowing, embrittlement or outright decomposition of such materials. For this reason, ultraviolet screening agents are frequently included in paint and plastics so as to prevent significant penetration of ultraviolet radiation therethrough. In other instances, as for example in display windows and the like, ultraviolet absorbing screens or filters are employed to prevent damage to the displayed goods. In addition to damaging goods, ultraviolet rays have been identified as being destructive to human tissue, particularly as causing of skin cancer and retina damage in humans. Therefore, it has been found desirable to limit exposure to such harmful radiation as by the inclusion of ultraviolet filters in architectural glazing panels, automobile and airplane windows and the like.
It is frequently desirable to also filter out the infrared wavelengths of the solar spectrum. Although infrared photons are of relatively low energy and therefore unable to break many chemical bonds, they are of a wavelength particularly well adapted to induce molecular vibrations, thereby heating materials upon which they impinge. In some instances, such as in greenhouses and solar collectors for example, it is desirable to maximize the transmission of infrared radiation so that said radiation may be absorbed in a medium for the production of heat. However, in other instances it is desirable to prevent or at least reduce the transmission of infrared radiation so as to, for example, lessen the burden on air conditioning systems.
It may thus be seen that it is necessary to utilize optical filtering mechanisms in order to limit the passage of harmful and/or unwanted solar radiation into particular areas. Furthermore, and as should be obvious from the FIG. 1 depiction of the various colors of the visible wavelengths of the solar spectrum, transmission of preselected wavelengths for aesthetic and/or practical reasons may be maximized by using an optical filter to judiciously absorb, reflect or transmit portions of incident radiation.
Optical filters typically operate by either absorbing or reflecting portions of light incident thereupon. In absorption-type filters, chromophoric materials such as dyes, metal ions and the like are adapted to absorb selected energies of incident light. By judicious design of a chromophoric material, the wavelength of the transmitted light may be selected, and by judicious selection of the concentration of chromophoric material, the intensity of the transmitted light may be selected.
Reflection filters typically operate on the principle of constructive interference. In such reflection filters, layers of the appropriate materials, having thicknesses which are precise multiples of preselected wavelengths of light, interact with rays of those preselected wavelengths to either facilitate the transmission or reflection of those particular wavelengths. By exercising appropriate control of the thickness and materials from which these layers are fabricated, the transmission characteristics of the filter may be rather precisely controlled.
Heretofore, the light which was not transmitted by prior art optical filters was effectively wasted. In absorption type filters, the chromophoric material absorbs incident photons from the solar spectrum and therefore becomes warm. This warming effect is at the very least wasteful of incident light energy and in some cases is actually detrimental or damaging to the operation of the filter. For example, many buildings and automobiles employ tinted glass optical filters for purposes of maintaining a lowered ambient temperature therewithin. In such instances, heating of the tinted glass which results from the absorption of incident photons is counterproductive insofar as that heat is at least partially radiated into the enclosed environment. In other instances, the absorption of light by the chromophoric material causes degradation of that material, which degradation is manifested by a change in optical properties of the material. In some cases the build up of heat in a light absorbing optical filter may be so severe as to melt, crack or otherwise deform the filter, thereby rendering it useless. While the problem of heat buildup is obviously not significant in interference-type filters insofar as such filters reflect, rather than absorb, radiation which is not transmitted therethrough; the reflected light is effectively wasted and can go so far as to actually constitute a nuisance, as for example, when the light is reflected from one building onto adjoining property.
As should be obvious from the foregoing discussion, optical filters have gained widespread popularity and importance, particularly as glazing materials for automobile windshields and architectural structures. This popularity and utility stems from the fact that said filters provide mankind with control of the ambient flux of light of aesthetic and/or energy conservation purposes. Furthermore, it should be apparent that prior art optical filters suffered from shortcomings because, as detailed hereinabove, the light which is not transmitted through heretofore available optical filters oftentimes either damaged those filters or presented a nuisance to adjoining property.
On a different, but interrelated subject; due to the fact that there is an increasing world demand being placed upon ever dwindling non-renewable resources, energy prices are now at a premium. New sources of energy and new methods of energy conservation are being eagerly sought. Glass covered portions of buildings, particularly modern high rise buildings, and automotive windshields present relatively large areas which are exposed to incident solar radiation. Prior to the subject invention, this radiation was effectively wasted, at times detrimentally, by heretofore available optical filter materials. It would clearly be of great advantage to effectively use this "waste light" for purposes of productive power generation.
Photovoltaic devices have enjoyed increasingly greater use for the generation of power insofar as they are inherently non-polluting, silent and consume no expendable natural resources in their operation. However, until recently, photovoltaic devices were fabricated from single crystal materials which severly limited the utility of such devices insofar as crystalline materials are difficult to produce in large areas are relatively thick, fragile and heavy and are expensive and time consuming to fabricate.
Recently, considerable efforts have been made to develop processes for depositing thin film semiconductor materials which can encompass relatively large areas which can be readily doped to form p-type and n-type as well as intrinsic materials for the production of photovoltaic devices substantially equivalent to those produced by crystalline materials. Among such thin film materials are amorphous materials and it is to be noted that the term "amorphous" as used herein, includes all materials or alloys which have long range disorder, although they might have short or intermediate order or even contain, at times, crystalline inclusions.
It is now possible to prepare by glow discharge or other vapor deposition techniques, thin film amorphous silicon or germanium based alloys in large areas, said alloys possessing acceptable concentrations of localized states in the energy gaps thereof and high quality electronic properties. Suitable techniques for the fabrication of such materials are fully described in U.S. Pat. No. 4,226,898, entitled "Amorphous Semiconductor Equivalent to Crystalline Semiconductors," of Stanford R. Ovshinsky and Arun Madan which issued Oct. 7, 1980 and in U.S. Pat. No. 4,217,374 under the same title which issued on Aug. 12, 1980, to Stanford R. Ovshinsky and Masatsugu Izu and in U.S. Pat. No. 4,504,518 of Stanford R. Ovshinsky, David D. Allred, Lee Walter and Steven J. Hudgens entitled "Method of Making Amorphous Semiconductor Alloys and Devices Utilizing Microwave Energy," which issued on Mar. 12, 1985, and in U.S. Pat. No. 4,517,223 under the same title which issued on May 14, 1985 to Stanford R. Ovshinsky, David D. Allred, Lee Walter and Steven J. Hudgens, the disclosures of which are incorporated herein by reference. As disclosed in these patents, it is believed that fluorine introduced into the amorphous semiconductor operates to substantially reduce the localized states therein and facilitates the addition of other alloying and/or dopant materials.
Unlike crystalline silicon, amorphous silicon and germanium alloys can be deposited in multiple layers over large area substrates to form semiconductor devices such as solar cells in a high volume, continuous processing system. Such continuous processing systems are disclosed in the following U.S. Pat. Nos.: No. 4,400,409, for "A Method of Making P-Doped Silicon Films and Devices Made Therefrom," U.S. Pat. No. 4,410,588, for "Continuous Amorphous Solar Cell Deposition and Isolation System And Method," U.S. Pat. No. 4,542,711 for "Continuous Systems For Depositing Amorphous Semiconductor Material" U.S. Pat. No. 4,492,181 for "Method And Apparatus For Continuously Producing Tandem Amorphous Photovoltaic Cell" and U.S. Pat. No. 4,485,125 for "Method And Apparatus For Continuously Producing Tandem Amorphous Photovoltaic Cells". As disclosed in these patents, the disclosures of which are incorporated herein by reference, a substrate may be continuously advanced for deposition of semiconductor layers thereonto through a succession of deposition chambers, wherein each chamber is dedicated to the deposition of a specific semiconductor material. For example, in making a solar cell of n-i-p type configuration, a first chamber is dedicated to depositing an n-type amorphous silicon alloy, the second chamber is dedicated to depositing an intrinsic amorphous silicon alloy, and the third chamber is dedicated for depositing a p-type amorphous silicon alloy.
The layers of the semiconductor material thus deposited in the vacuum envelope of the deposition apparatus may be utilized to form a photovoltaic device including one or more p-i-n cells, or one or more n-i-p cells, a Schottky barrier, as well as other semiconductor devices. Additionally, by making multiple passes through the deposition chambers, or by providing an additional array of deposition chambers, multiple stacked photovoltaic cells, or photovoltaic cells having component layers fabricated from a large number of stacked thin film layers, may be obtained.
It may thus be seen that thin film photovoltaic technology has now matured to a point where large area, high efficiency photovoltaic cells may be readily and economically manufactured. Thin film photovoltaic materials are ideally suited for use in fabricating optical filters which have been specially tailored so as to generate power from the energy present in that non-transmitted portion of the incident solar spectrum referred to hereinabove as "waste light". Thin film semiconductor alloy materials may be readily deposited in large areas upon a wide variety of conventionally and unconventionally configured substrates. Additionally, the optical absorption, band gap, transmittance and other physical properties of said thin film semiconductor alloy materials may be readily controlled by the techniques described in the patents incorporated herein by reference so as to provide a thin film photovoltaic body having desirable light absorption and transmission characteristics. Furthermore, by utilizing the deposition techniques referred to herein, structures comprised of a multiplicity of thin film layers of preselected wavelengths may be fabricated so as to enhance the optical absorption and/or transmission of those materials for various portions of the solar spectrum. For these reasons, it should now be apparent that optical filters may be fabricated by utilizing thin film semiconductor methods and techniques referred to herein, which filters (1) exhibit a desirable, preselected optical transmission and/or absorption and (2) effectively utilize a portion of preselected wavelengths of the non-transmitted light for the productive generation of electrical power.
Because of the inherent limitations of single crystalline materials, it is impossible to fabricate power generating optical filters therefrom. More particularly, single crystal materials are limited in size by the inherent difficulty of fabricating a perfect crystal and for this reason cannot be readily utilized in large area applications (such as architectural glass). Secondly, single crystal materials are relatively thick and brittle and therefore cannot be processed in thicknesses which would allow for significant light transmission therethrough; nor may they be made to conform to irregularly shaped surfaces (such as automotive windshields). And thirdly, single crystalline materials are fixed in composition and stoichiometry and therefore cannot have their optical properties modified so as to provide optical transmission of preselected wavelengths of incident radiation.
According to the principles disclosed herein large area, optical filters having preselected transparencies to various wavelengths of the incident solar spectrum may be readily fabricated by the use of a plurality of layers formed of thin film semiconductor alloy materials so as to provide the dual function of light filtration and power generation. Such filters have wide utility in the manufacture of architectural glazing panels, automotive windshields, optical filter elements such as lenses, and the like.
These and other advantages of the instant invention will be readily apparent from the brief description, the drawings and the description of the drawings which follow.